In some audio speakers, a Motion Feed Back (“MFB”) circuit is mounted as a technique to improve the sound quality of the speaker. The MFB circuit detects the operating state of a vibrating diaphragm through an electric signal showing audio information (hereinafter referred to as an “audio signal”) that is inputted to a speaker, and feedback-controls the diaphragm based on the detected result. The distortion of sound, which is especially likely to be occurred in a low tone range, can be canceled. Therefore, it is sometimes mistakenly assumed that the MFB circuit is effective to be utilized in a small-sized speaker in which reproduction in a low tone range is difficult.
For example, the following five references with regard to an MFB circuit are known; Japanese Patent Laid-Open No. Sho 52-79644, Japanese Patent Laid-Open No. Sho 53-12319, Japanese Patent Laid-Open No. Sho 53-12320, Japanese Patent Laid-Open No. Sho 53-12321, and Japanese Utility Model Laid-Open No. Sho 57-96589. In these references, the operating state of the diaphragm is detected by detecting the variation of an electrostatic capacity formed between electrodes. More specifically, an electrode (hereinafter, referred to as “movable electrode”) is fixed to a diaphragm or to an electromagnetic coil which is referred to as a voice coil bobbin and that causes the diaphragm to vibrate, and another electrode (hereinafter, referred to as “fixed electrode”) is fixed so as to face the movable electrode. An electrostatic capacity, which is varied by the movable electrode being moved relative to the fixed electrode, is detected and outputted as a detection signal. After that, the detection signal corresponding to the electrostatic capacity and an audio signal are compared with each other by a comparison device (for example, a CPU), and then the operation of the diaphragm is appropriately controlled on the basis of the compared result, i.e., the difference between the output level of the detection signal and the output level of the audio signal.
However, the electrostatic capacity that is formed between the electrodes is very small, for example, from several picofarad (pF) to several hundred of pF. Therefore, the electrostatic capacity is affected and varied by a small amount of electromagnetic waves, static electricity, or the like. For example, a diaphragm is commonly structured to vibrate by an excitation effect between a voice coil bobbin, an iron core that is inserted into the voice coil bobbin and referred to as a center pole, and a magnet that generates a magnetic flux passing through the voice coil bobbin and the center pole. However, the electrostatic capacity between the electrodes is affected and varied by an exciting current flowing through the voice coil bobbin. Further, some of electronic components that are incorporated into a speaker emit an electromagnetic wave although it may be weak, and the electrostatic capacity may be varied by the electromagnetic wave that transmits to the electrodes. Further, the electrostatic capacity between the electrodes may be affected by friction accompanied with mechanical phenomena such as vibration of components which are incorporated in the speaker, static electricity caused by various electromagnetic phenomena in the inside and the outside of the speaker, electromagnetic waves which are outputted by electronic equipment installed around the speaker, or the like. As described above, in the above-mentioned prior art, the electrostatic capacity varies and the electrostatic capacity formed between the electrodes is unable to be accurately detected.
Further, a movable electrode used in the above-mentioned references is made of a metal foil. The metal foil is moved in a reciprocating manner in a constant magnetic field to generate an eddy current and thus a correct electrostatic capacity is not obtained. Therefore, an operating state of the diaphragm cannot be accurately detected and thus distortion of a sound in a low tone range is not sufficiently reduced.